The object of the present invention is a method for use in connection with closing a liquid packaging container, the said container comprising a cylindrical or truncated cone-like casing part and end members connected to it, of which at least one is formed of fibre-based, such as board-based material, for example, liquid packaging board, and comprises a skirt folded in the direction of the casing part, by which skirt the end member is connected to the casing, in which method the container, which is finished at least as concerns its casing part and the said one end member, is filled with a liquid and the container is closed.
The conditions relating to the filling of a liquid container often differ from its storage conditions, which means that such a high underpressure may form inside the container that the casing part of the container provided with a cylindrical or truncated cone-like casing part may buckle inwards. The formation of underpressure is due particularly to the fall in the temperature of the gas remaining in the container during filling, and to chemical reactions between gases and the liquid being packaged. The cooling of the liquid also contributes to the formation of an underpressure, although this effect is relatively minor. The buckling problem occurs especially in containers, whose casing part is made of a board-based material, that is, of so-called liquid packaging board, in which case the rigidity of the material is not sufficient to prevent buckling.
Factors causing the formation of underpressure in the empty space in the container include the temperature of the gas in this space at the time of closing the container, the degree to which the container is filled, the temperature of the product being packaged, the temperature of the space where the container is stored, and the product being packaged.
From U.S. Pat. No. 4,338,765 a method is known for solving the problem described above. In this method, when the container is being filled and closed, the base of the container is moved in and out in the manner of bellows by means of a suitable plunger, air blast or suction in order to compensate for the internal pressure changes in the container while packaging liquid which is hot at the time of packaging in the container. It is proposed to use paper, aluminium foil, plastic sheet, or a combination thereof, onto which a thermoplastic resin is laminated or coated, as the material for the base which is to be forced to move in a bellows-like fashion. Only as relatively thin films are these materials sufficiently elastic for the application of the method. Such thin films are not, however, adequate for making the base of the container strong enough mechanically to withstand the potential forces exerted on the container in the environment where it will be used or transported, for example, in a retail shop shopping trolley. In practice, when applying the method relating to the said patent, it has been necessary to provide the container with a second base made of board material, which acts only as mechanical protection.
The buckling of the casing part of the container due to the combined effect of the above-mentioned problems is a phenomenon that is relatively difficult to eliminate. The aim of the present invention is, however, to present a method for use in connection with closing a liquid container, by means of which the buckling inwards of the casing part of a liquid container during storage can be prevented. A further aim is to present a method, the application of which makes it unnecessary to provide the container with a separate base acting as mechanical protection, in addition to the thin pressure-balancing base.
These aims are achieved by the method relating to the invention, which is characterised in that the one end member, which has already been incorporated in the container when it is filled, is moistened on its surface outside the container before closing the container. The moistening is preferably carried out by directing a steam jet at the end member. In moistening with steam, the moisture is able to penetrate into the fibre-based material of the end member, which means that the rigidity of the end member is reduced substantially at least for a time, which in turn allows the end member to be elastic when the container is being filled. If, when filling the container, and utilising the elasticity thus achieved, the end member is sucked outwards from its perhaps somewhat convex shape in the direction of the interior of the container, to a perhaps somewhat outwards convex shape, it will be possible, on the one hand, to fill the container to a fuller degree than would otherwise be possible, as a result of which the head space remaining in the container once the end member has returned to its normal position, after the container has been closed, becomes smaller than usual, and above all the bellows-like movement inwards of the end member increases the pressure of the gas in the head space, thus compensating for the fall in pressure following the cooling down of the gas. Significant cooling down of the gas and the fall in pressure following it naturally occur when the liquid packaged in the container is relatively hot.
An alternative way in which the container functions, in which the moistening method relating to the invention is also needed, is illustrated by a situation where the weight of the liquid packed in the container is allowed, either alone or together with external suction, to press the base of the container at least to some extent outwards into a convex shape. In this case, once the container has been closed, the base will not return to its original position, but perhaps partly. A fall in the pressure of the gas remaining in the container while it is being closed may in this case draw the base of the container towards the interior of the container during storage. The base thus acts as a pressure-balancing means and prevents the buckling of the container casing.
The method relating to the invention may be supplemented by a method stage in which the said one end member is shaped before attaching it to the container casing to comprise at least one fold or the like which runs around the end member, close to its skirt. This type of folding or corrugation provides the end member with additional elasticity which can be utilised when the rigidity of the end member is reduced by means of steam moistening.
The elasticity can also be increased in such a way that, when attaching the said one end member to the container casing, the attachment is only done over a part of the length of the skirt. In this way the fold point between the end member and the skirt acts in the same way as the folding or corrugation described above, and gives the end member additional elasticity.
When applying the method relating to the invention, the container may be closed either by attaching one of its end members to it, or alternatively, if filling has been carried out through a filling aperture in one of the end members, the container may be closed by attaching a closure flap over this filling aperture.